1. Field of the Invention
This invention relates to a method for controlling the equivalent water content of a catalyst stream comprising water and a strong acid. More specifically, this invention concerns a method for continuously monitoring the equivalent water content of a hydrocarbon conversion catalyst, particularly an alkylation catalyst, comprising water and fluorosulfuric acid by adding SO.sub.3, e.g. fuming sulfuric acid, to a sample of said catalyst at a rate sufficient to maintain the mixture thus formed at the point of incipient fuming and detecting the presence of the SO.sub.3 evolved therefrom. The equivalent water content of the catalyst sample can then be determined from the flow rate of the sample and the flow rate and composition of the fuming sulfuric acid. The equivalent water content of the catalyst stream being monitored can then be controlled by regulating the addition rate of fresh acid, water or both to the hydrocarbon conversion process in accordance with the deviation between the equivalent water content and the desired equivalent water content of the catalyst sample.
2. Description of the Prior Art
The alkylation of paraffins with olefins in the presence of a catalyst comprising fluorosulfuric acid is well known in the petroleum refining art, see, for example, in U.S. Pat. No. 3,887,635, the disclosures of which are incorporated herein by reference. Typically, in such processes a paraffin, preferably an isoparaffin such as isobutane, and olefins such as butenes are alkylated in the presence of a catalyst formed from fluorosulfuric acid and water according to the following equation: EQU HSO.sub.3 F + XH.sub.2 O .revreaction. XHF + XH.sub.2 SO.sub.4 + (1-X) HSO.sub.3 F (1)
where X represents the moles of water per mole of fluorosulfuric acid employed to form the catalyst. The water serves to produce a catalyst of reduced acidity vis-a-vis the fluorosulfuric acid, and thereby decreases the probability of undesirable competing side reactions which have a detrimental effect on product quality, while increasing catalyst selectivity to desirable highly branched paraffinic products to form alkylate product of high quality. Since the composition of the active catalyst and the selectivity of same will depend upon the amount of water employed, it would be desirable to have available simple and convenient method of determining the water equivalent to that used to form the desired catalyst.
A method has been suggested for determining the water content of a sulfuric acid catalyst in which fuming sulfuric acid is mixed with the acid catalyst until fuming is initiated (see Albright, L. F. et al, "Alkylation of Isobutane with Butenes; Effect of Sulfuric Acid Compositions," Ind. Eng. Chem. Process Des. Devlop. Vol. II (No. 3), p. 446-450, 1972). However, it is believed that this method does not suggest controlling continuously the equivalent water content of a fluorosulfuric acid catalyst as is described herein below.